Process for applying thin molybdenum containing coatings on aluminum for solar energy absorption

ABSTRACT

A thin, adherent coating is described that is highly absorptive to solar energy and has a low emissivity for thermal energy, thereby being a useful selective surface for solar energy collection. This coating is formed on aluminum and its alloys by a simple electrochemical process and consists of complex aluminum and molybdenum oxides and metallic molybdenum which are deposited as a near-monatomic layer of molybdenum and its oxides. The coating withstands exposure to 400° F (204° C) and 1 hour in boiling water without change in properties.

This is a division, of application Ser. No. 615,245, filed Sept. 22,1974 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to solar heating systems and more particularlyrelates to efficient absorption and retention of solar energy. Itespecially relates to coatings, to be applied to aluminum surfaces,having high selectivity for solar energy absorption.

The concept of using solar energy as a source of heat is probably as oldas mankind itself and the usual procedure was to heat a black surface bythe rays of the sun in order to extract heat therefrom.

Solar radiation reaching the surface of the earth is mainly concentratedin the visible spectrum and does not exceed a wave length of about 2microns. On the other hand, the long-wave thermal radiation spectrum isgreater than 3 microns so that there is virtually no overlapping of theshortwave solar spectrum and the longwave thermal spectrum. Because ofthis rather recent realization, the concept of utilizing selectivesurfaces for solar collectors has received added attention. Selectivesurfaces are those whose absorptance and emittance vary with the wavelength of the incident radiant energy. Thus, in order to be a good solarcollector, a surface exposed to solar radiation must be as littlelight-reflecting and as little transparent to light as possible;that-is, as dark as possible, preferably black. However, it cangenerally be stated that the darker a body, the more heat it radiateswith increasing temperature, the heat radiation having wave lengthsabove about 2 or 3 microns. Therefore, the better the receiver isadapted to absorb solar radiation, the greater the energy heat losses byheat radiation with the result that it had not been possible untilrecently to obtain utilizable energy at high temperature from solarheaters, except by the use of optical systems of high concentrationpower.

Black paint has long been the most utilized of the non-selectivecoatings, that is, it is a coating whose absorptance and emittanceremains constant over the entire spectrum of thermal and solar waves.Black paint performs reasonably well up to a collector temperature ofabout 150° C., but above this temperature, its efficiency declines dueto thermal losses from re-radiation. Paint also has the seriousdisadvantage of being an organic material subject to deterioration undernormal weather and solar conditions. The main advantage of black paintcollectors is their ease of manufacture and low cost.

Recently, there has been made available to the art solar coatings whichare quite selective, i.e., materials that have a high absorption forsolar radiation and at the same time exhibit very low emissive lossesfor thermal radiation.

One such coating involves aluminum or an alloy thereof which has beenanodized and thereafter blackened by reaction therewith with a coppersalt solution. A coating of this type is disclosed in U.S. Pat. No.2,917,817. This coating is stated to be selective and particularlyadapted to be used as a solar collector.

However, there are other physical characteristics of a solar coatingwhich must be taken into consideration aside from its absorptance andemittance characteristics. Thus, a solar coating should be stable. Itshould not degrade with time and it should be able to be easilyreproducible so that it is adaptable for commercial manufacture.

It is precisely in this area that the heretofore employed prior artcoatings have suffered serious drawbacks. Thus, for example, it is knownin the art that the α and ε characteristics of certain coatings,including aluminum oxide coatings, are directly related to the thicknessof the coating. By way of considerable oversimplification, it can bestated that a thick oxide coating is undesirable from the point of viewof its α and ε characteristics. However, when using a thin oxide coatingproduced by anodizing aluminum, severe mechanical properties havearisen, particularly when the necessary blackening solution wasemployed. A thin anodized layer of aluminum oxide which has beenblackened is subject to irregularities in the surface characteristicsthereof, thereby detracting from its potential for use as a solarcollector.

Relatively hard, porous, adherent, and adsorbent coatings of such metalsas copper, silver, and gold have been formed on aluminum by firstanodically oxidizing an aluminum article in an electrolyte such assulfuric acid, chromic acid, or oxalic acid and then depositingcompletely reduced metal thereon, as disclosed in U.S. Pat. No.1,988,012. However, lighter colored silver deposits include unreduced orpartially reduced metal salts so that the coatings change color whenexposed to sunlight.

For present purposes, the term "aluminum" is used with reference to themetal itself and alloys composed predominantly of aluminum, includingcommercial grades of aluminum with ordinary impurities and normalwrought alloys which may also contain added elements in relatively minoramounts, especially those alloys containing upwards of 90% aluminum byweight.

One measure of the selectivity of a particular surface is the ratio ofthe shortwave solar absorptance to the longwave thermal emittance. Thisratio, however, does not always adequately define the bestabsorptance-emittance characteristics for a practical solar collector.Thus, for instance, various surfaces of sufficiently low emittance mayhave high α/ε ratios and yet exhibit only moderate absorptance. It hasbeen found desirable, therefor, to consider the difference factor (α-ε)as an additional indicator.

Among the best state-of-the art solar coatings are black nickel andblack chrome, which typically have an α/ε ratio of about 10:1. They aretechnically quite suitable, but rather expensive to produce. Othersurfaces also exhibiting high α/ε ratios sometimes do not have adequateproperties to qualify as practical solar coatings. Polished zinc, forexample, may have an absorptance of 0.5 and emittance of 0.05, giving anα/ε ratio of 10:1. Similarly, bare aluminum exhibits typical α/ε valuesup to 15:1 depending on surface condition. Neither of these surfaces hasan acceptably high absorptance even though their α/ε ratios are verygood.

Absorptance values are conveniently measured using a Gardner ModifiedHazemeter with a filter giving the maximum intensity at about 5560 A,the maximum visible wavelength. Emittance may be determined at 300° F.,using a Gier Dunkle Total Normal Emittance system.

The natural oxide of aluminum formed on its exposed surfaces is not adesirable attribute for solar heat receptor devices because suchsurfaces, although having a rather high absorptance/emittance ratio(α/ε), tend to exhibit a low absorptance of only about 0.40, i.e., theirhigh α/ε ratio comes about only by virtue of an even lower emittancecharacteristic. Moreover, the difference (α-ε) between their absorptanceand emittance is typically less than 0.40.

It has been recognized that absorptance can be increased appreciably (atleast doubled) by use of black paint on aluminum surfaces. However, theα/ε ratio obtained is closer to unity, since the emittance of suchpainted surfaces is about the same as their absorptance.

Other ways have been proposed to solve this problem, in order to be ableto exploit other desirable properties of aluminum, especially its goodheat conductivity, ease of forming into fabricated articles, andrelatively low cost. Perhaps the most successful, although quite costly,is to use plating operations to deposit one or more layers of nickel orother metals of suitable properties.

Thus, for example, nickel on aluminum is effective to achieve a moderateabsorptance of about 0.55 and emittance of about 0.15 (or typically α/ε= 3.7 and α-ε = 0.40); polished zinc on aluminum achieves about the sameabsorptance and lower emittance, hence a higher α/ε ratio (typically10:1) but about the same difference (α-ε). So-called black nickel andblack chrome show higher absorptance (0.87 to 0.88) and low emittance(0.07 to 0.11) for a typical (α/ε) ratio of 10.0:1 and a typical α-εdifference of 0.8. Black anodized aluminum has excellent absorptance butrelatively high emittance (α/ε about 1.2, with α-ε of only about 0.20).As has heretofore been mentioned, its absorptance and emittancecharacteristics can be improved only at the expense of its mechanicalproperties.

From the foregoing, it may be noted that various approaches have beenavailable for increasing absorptance or decreasing emittance, but thoseable to do both tend to be prohibitively expensive for routine use.

Based on the α/ε parameter, a black nickel coating, mill-finishedaluminum, and polished zinc all appear to be suitable and much betterthan some others. Using the α-ε parameter, however, the relative meritsof various surfaces are more readily apparent.

The basic structural elements of apparatus suitable for purposes of theinvention are a sheet or panel of relatively high surface area comparedto other dimensions, usually having fins or the like to increase theeffective area, and means such as tubular conduits formed integrally orotherwise disposed in heat exchange relationship therewith fortransmitting fluid to extract heat. The fluid used is normally water oran aqueous solution of lower freezing point, but it may also be a gassuch as air.

For purposes of background information on related aspects of solar heatcollection, reference is made to "Low Temperature EngineeringApplication of Solar Energy," published by American Society of Heating,Refrigerating and Air-Conditioning Engineers, Inc. (especially ChapterIV on Selective Surfaces for Solar Collectors); and, on the subject ofchemical oxide coatings for aluminum to "The Surface Treatment andFinishing of Aluminum and its Alloys," by Wernick and Pinner. 1959,published by Robert Draper Ltd. (especially section 5 on ChemicalConversion Coatings, beginning at page 166).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of this invention produces a thin, adherent coating onaluminum and its alloys that is highly absorptive to solar energy andhas a low emissivity for thermal energy. It is, in other works, aselective surface having a high ratio (α/ε) of short-wave solarabsorptance to long-wave thermal emittance; it also has a highdifference factor (α-ε). Aluminum and aluminum alloys having the coatingof this invention are thus highly useful as collector surfaces in solarcollectors for heating and cooling homes and other buildings.

This coating consists of complex aluminum and molybdenum oxides andmetallic molybdenum as a near-monatomic layer of molybdenum which isdeposited along with these oxides. Molybdenum exhibits the phenomenon of"self-polarization;" that is, it will not deposit on itself, so that assoon as a monatomic layer is formed, deposition ceases unlessdepolarization comes about through simultaneous deposition of anothermetal or by the formation of an alloy with the cathode.

In order to produce coatings having a selected thickness, whereby theabsorptance and emittance of selected wavelengths of light and of heatradiation can be respectively controlled, depolarizers such as nickelmay be added to the molybdenum electrolyte. Coatings with increasedresistance to severe heat and humidity are produced by utilizing animmersion zinc coating prior to the molybdenum coating. Likewise, zinc,when present in the alloy, tends to stabilize the black molybdenumcoating against change resulting from severe heat and humidity.

The process of this invention comprises electrochemically plating aclean aluminum surface in a solution containing ammonium molybdate atroom temperature. A satisfactory solution is slightly alkaline andcontains MoO₃, and NH₄ OH. A cleaned aluminum surface, immersed for afew minutes at low current density, produces a coating on the aluminumhaving high absorptivity and low emissivity at elevated temperatures.

Time, temperature, and current density are not critical because thecoating tends to be self-limiting in thickness due to theself-polarization of molybdenum. Thereafter, the rate of depositionrapidly declines. As a result, a near-monatomic layer of molybdenum isdeposited along with this oxide. This feature insures constant,reproducible results. The coating withstands exposure to 400° F and onehour in boiling water without change in properties.

Time and current density are interrelated in that a high current densityrequires less time and low current density requires a longer time toachieve an acceptable absorptance. Plating should be stopped after anacceptable absorptance has been obtained because even though the coatingtends to be self-limiting, there is some additional deposition onprolonged treatment that tends to increase the emittance. Acceptablecoatings have been produced at current densities from 2 to 12 amperesper square foot and at coating times from 1 to 6 minutes.

The ammonium molybdate used may be either the commercially preparedammonium molybdate or can be formed in situ from molybdenum oxide andammonium hydroxide. The solution pH can be from acid to highly alkaline;however, the preferred range is 8-10. The solution temperature seem tohave no effect on the formation of the coating, and room temperature isused only for convenience. The concentration of the molybdate solutionis not critical. Successful coatings have been formed in solutionscontaining only 7.5 grams/liter MoO₃ and 5 ml/l NH₄ OH.

This process is effective on aluminum and aluminum-alloy articles havingsubstantial thickness and is also useful on thin aluminum sheets and onaluminum foils for solar heating applications such as those described inU.S. Pat. No. 3,129,703.

EXAMPLE 1

A cleaned aluminum article was plated by immersing it with power on asthe cathode in a solution at room temperature for two minutes at 3amperes per square foot. The solution contained ammonium molybdate madeup with 30 grams/liter of MoO₃ and 20 ml/liter of NH₄ OH at pH 9.

The coating on the aluminum article was tested on the Gardner-ReynoldsModified Hazemeter and the Gier Dunkle Total Normal Emittance System andfound to have an absorptance for visible light of 0.913 and on emittanceat 300° F of 0.158, the α/ε ratio thus being 5.8 and the α-ε differencebeing 0.75. This coating was subjected to boiling water for 1 hour afterwhich the absorptance was found to be 0.924 and the emittance 0.143,giving an α/ε ratio of 6.5 and a α-ε difference of 0.781. The coatingwas extremely thin, and examination by ISMA (Ion Beam Surface MassAnalyzer) showed it to be composed mainly of molybdenum and molybdenumoxide.

The aluminum article was then immersed in boiling water for 1 hour andagain tested; its absorptivity and emissivity properties were unchanged.The article was placed in an oven at 400° F for 1 hour. When exposed tosolar radiation, its properties were again found to be unchanged.

EXAMPLE 2

A similar cleaned aluminum article was immersed in the same solution atroom temperature for 3.5 minutes at 3 amperes per square foot. Theplating rate was observed to rapidly decline as evidenced by increasinggassing at the cathode, after about 2 minutes plating.

The article was tested for solar absorption and thermal emittance andwas found to have substantially the same absorptivity and emissivity asin Example 1. The coating was also found to be substantially monatomicin thickness.

EXAMPLE 3

A cleaned aluminum article was given a standard double zincate immersioncoating, as commonly practiced in the plating industry, and then platedin a solution containing 30 g/l MoO₃ and 20 ml/l NH₄ OH at roomtemperature and at pH 9, for 2 minutes at 3.5 amperes per square foot.Upon testing, the properties of this coating were 0.913 absorptance and0.2 emittance. After 1 hour in boiling water, the specimen was retestedand found to have 0.906 absorptance and 0.182 emittance.

EXAMPLE 4

Small solar collector modules were prepared for testing in a desertlocation. Test coupons were run with each module. The electrolyte usedwas made up with 30 grams per liter of commercially available ammoniummolybdate with the pH adjusted to 9.0 with NH₄ OH.

Module 1 was cleaned and then plated for 3 minutes at 3 amperes persquare foot. The test coupon showed an absorptance of 0.913 and onemittance of 0.136.

Module 2 was cleaned, given the standard double zincate treatment, andthen plated for 1.5 minutes at 3 amperes per square foot. The testcoupon showed properties of 0.906 absorptance and 0.182 emittance.

Because it will be readily apparent to those skilled in the art thatinnumerable variations, modifications, applications, and extensions ofthese embodiments and principles can be made without departing from thespirit and scope of the invention, what is herein defined as such scopeand is desired to be protected should be measured, and the inventionshould be limited, only by the following claims.

What is claimed is:
 1. The process of producing a thin, adherent coatingon an article of aluminum and alloys thereof that is highly absorptiveto solar energy and has a low emissivity for thermal energy, saidprocess comprising:A. providing a clean surface on said article; B.immersing said surface in a solution containing ammonium molybdate; C.connecting said article as the cathode in a direct-current circuit; andD. electrochemically plating said clean surface to form a coating whichis a nearmonatomic layer of molybdenum and oxides thereof.
 2. Theprocess of claim 1 wherein said solution is slightly alkaline andcontains MoO₃ and NH₄ OH.
 3. The process of claim 2 wherein an immersionzinc coating is deposited prior to said plating.
 4. The process of claim3 wherein nickel salts are added to said solution as depolarizers priorto said plating, whereby coatings of selected thickness are produced. 5.The process of claim 2 wherein said solution consists of 30 grams ofliter of MoO₃ and 20 milliliters per liter of NH₄ OH at pH
 9. 6. Theprocess of claim 2 wherein said article is immersed in said solution for1-6 minutes at low current density.
 7. The process of claim 2 whereinsaid current density is about 2-12 amperes per square foot.